added requirements.txt with known good aiohttp version for chapter 17 examples
added requirements.txt with known good aiohttp version for chapter 17 examples

Wallace-dyfq/example-code,fluentpython/example-code,everaldo/example-code,everaldo/example-code,friebsch/example-code,Wallace-dyfq/example-code,Wallace-dyfq/example-code,fluentpython/example-code,everaldo/example-code,dcbdmb/example-code,friebsch/example-code,everaldo/example-code,dcbdmb/example-code,oxfordyang2016/learnfluentpython,fluentpython/example-code,oxfordyang2016/learnfluentpython,friebsch/example-code,oxfordyang2016/learnfluentpython,dcbdmb/example-code,Wallace-dyfq/example-code,oxfordyang2016/learnfluentpython,fluentpython/example-code,dcbdmb/example-code,friebsch/example-code,oxfordyang2016/learnfluentpython,Wallace-dyfq/example-code,everaldo/example-code,dcbdmb/example-code,friebsch/example-code,fluentpython/example-code